North Korea has allowed the mother of Kenneth Bae to visit her ailing son almost a year after the American tour operator and missionary was detained and sentenced to 15 years hard labour for "hostile acts".

Myunghee Bae saw her son at a hospital in Pyongyang where he's being treated for various ailments. She said he had told her his health was improving but was "still not good".

"I came because I am worried that my son's health has worsened," she said when she arrived at the airport. "I really wanted to see my son." It is believed she will get to see him again during her five-day visit.

Bae said her trip had been approved by Washington.

In a video posted to the family's website before she left her home in the US city of Seattle, Myunghee Bae said her heart was "broken into pieces" when she saw a prison interview with her son from July. "He looked so different and he lost so much weight," she said. "I could not believe that prisoner was my son."

Bae was arrested while on a tour in a northeastern North Korean city last November and sentenced after a trial on April 30, further straining relations between the US and the government of Kim Jong-un.

North Korea has periodically detained Americans, sometimes seeking to barter their release for concessions from the US.

Bae worked in the fields at a labour camp until two months ago, when he was moved to a hospital because he had lost more than 20 kilograms. He also suffers from diabetes, an enlarged heart, liver problems and back pain, his family has said.

The US doesn't have diplomatic relations with North Korea, complicating efforts to work for Bae's release.

In August, Robert King, the State Department's special envoy on North Korean human rights, was due to travel to the country to seek Bae's freedom in what would have been the first visit by a US official in more than two years. North Korea withdrew the invitation at the last minute.

"We remain gravely concerned about Mr Bae's health and we continue to urge the DPRK authorities to grant Mr Bae special amnesty and immediate release on humanitarian grounds," Nolan Barkhouse, a spokesman for the US embassy in Beijing, said.

Bae is at least the sixth American detained in North Korea since 2009. The others eventually were allowed to leave without serving out their terms, some after prominent Americans visited the North.

Bae, a father of three, was born in South Korea and immigrated to the US with his parents and sister in 1985. He later moved to China, and a couple of years ago he began leading small tour groups to North Korea.

Bloomberg, Reuters

This article appeared in the South China Morning Post print edition as Mother meets ailing son in Pyongyang